Desirable properties for a heat collecting surface, where solar energy is collected as heat, include solar selective absorptivity, that is, a radiation spectrum specificity of the heat collecting surface which allows the maximum absorption of solar energy in the ultraviolet, visible and near infrared regions while minimizing the radiation loss from the surface in the infrared region which dominates the radiation spectrum.
Various techniques such as, for example, plating and coating have been known for applying a solar selective absorption treatment on a metallic heat collecting panel.
A plastic heat collecting panel, on the other hand, has been produced by blow molding a polyolefin resin, such as high-density polyethylene filled with carbon black or like material. However, it was difficult to apply a solar selective absorbing treatment on such plastic heat collecting panel.
There may be cited the following two reasons for which it has been difficult to achieve said solar selective absorption treatment on a polyolefin resin heat collecting panel.
The first reason is that the formation of a coating on polyolefin resin is extremely difficult because of the non-adhesiveness of such resin.
The second reason is that the conventional selective absorption treatments, which have been applied to metallic heat collecting panels all make use of the fact that the metallic heat collecting panels have a low thermal emittance. However, the techniques used for such metallic panels can not be applied to resin heat collecting panels because such resin panels have a high thermal emittance.